Split models.py into several files (#398)

* Move exception models into separate file
* Move thread model into separate file
* Move user model into separate file
* Move group and room models into separate file
* Move page model into separate file
* Move message model into separate file
* Move basic attachment models into separate file
* Move sticker model into separate file
* Move location models into separate file
* Move file attachment models into separate file
* Move mention model to reside with message model
* Move quick reply models into separate file
* Move poll models into separate file
* Move plan model into separate file
* Move active status model to reside with user model
* Move core enum model into separate file
* Move thread-related enums to reside with thread model
* Move typingstatus model to reside with user model
* Move emojisize and reaction enums to reside wtih message model

fbchat/_core.py

@@ -0,0 +1,12 @@
+# -*- coding: UTF-8 -*-
+from __future__ import unicode_literals
+
+import aenum
+
+
+class Enum(aenum.Enum):
+    """Used internally by fbchat to support enumerations"""
+
+    def __repr__(self):
+        # For documentation:
+        return "{}.{}".format(type(self).__name__, self.name)